In the present context, the term III-V nitride semiconductors encompasses the materials derived from GaN or related to GaN as well as, for example, ternary or quaternary solid solutions based thereon. It encompasses in particular the materials AlN, InN, AlGaN (Al1-xGaxN, 0≦x≦1), InGaN In1-xGaxN, 0≦x≦1), InAlN (In1-xAlxN, 0≦x≦1) and AlInGaN (Al1-x-yInxGayN, 0≦x≦1, 0≦y≦1).
In the text which follows, the term “III-V nitride semiconductors” relates to the above-described group of materials. Furthermore, this term comprises materials which are used to form buffer layers during the epitaxial production of layers belonging to the materials systems cited.
It is generally known that a semiconductor layer with few crystal defects can best be grown on a substrate with a lattice constant which is approximately equal to that of the semiconductor layer which is to be grown. However, a substrate material of this type which can be produced in particular at an economically viable level of technical effort is not available for semiconductor layers based on III-V nitride semiconductors (generally referred to below as III-V nitride semiconductor layer for short). Therefore, the only option at present is for semiconductor layers based on III-V nitride semiconductors to be grown on substrates such as sapphire, spinel or silicon carbide, but these materials generally have a different lattice constant than III-V nitride semiconductors.
If sapphire, SiC and similar substrates with different lattice constants are used, however, dislocations are formed in the epitaxy layer in a density of approximately 108 to 1010 cm−2. Charge carriers can recombine at such dislocations without emitting any radiation and are therefore no longer available, for example, for the generation of light in light-emitting diodes.
Therefore, it is already known from the prior art first of all to apply a buffer layer, for example of ZnO, to the substrate in order to reduce the difference in lattice constants between the III-V nitride semiconductor layer and the substrate. However, since the crystal properties of ZnO, for example on a sapphire substrate, are not of particularly good quality, it is consequently very difficult for a GaN semiconductor layer with good crystal properties to be grown on this ZnO buffer layer.
Therefore, by way of example, what is known as the ELOG process has been proposed for production of low-defect III-V nitride semiconductor layers. In this ELOG process, first of all a mask layer is applied to a substrate in order to form unmasked regions and masked regions on the substrate, the mask layer consisting of a material which substantially does not allow crystal growth of a semiconductor layer based on III-V nitride semiconductors. In a second step, a semiconductor layer based on III-V nitride semiconductors is grown epitaxially onto the unmasked regions of the substrate. As soon as the thickness of the III-V nitride semiconductor layer on the unmasked regions of the substrate exceeds the thickness of the mask layer, the III-V nitride semiconductor layer begins to grow together laterally over the masked regions until a completely continuous epitaxy layer has formed. Since the III-V nitride semiconductor layer therefore has mainly been formed not by growth on the substrate, but rather by lateral crystal growth, the continuous III-V nitride semiconductor layer has a significantly lower number of dislocations over the masked regions than over the unmasked regions of the substrate. The ELOG process can be used to produce III-V nitride semiconductor layers with a thickness of a few μm and a relatively low dislocation density.
Examples of an ELOG process of this type are disclosed in U.S. Pat. Nos. 6,153,010 A1 and 6,225,650 B1, the subject matter of which is hereby incorporated by reference.
The ELOG process was originally developed for a sapphire substrate and therefore has drawbacks in particular with other substrate materials. If the coefficient of thermal expansion of the grown III-V nitride semiconductor layer is greater than that of the substrate, as is the case, for example, with the combination of AlxGa1-xN on SiC, stress-induced cracks are formed in the epitaxy layer during the cooling phase from the growth temperature to room temperature, which may make the III-V nitride semiconductors unusable.